1. Field of the Invention
The present invention relates to an airway adapter which monitors both airway CO.sub.2 concentration and respiratory flow. More specifically, the present invention relates to an integrated airway adapter which is capable of monitoring CO.sub.2 concentration in real time, breath by breath, using infrared absorption techniques in combination with monitoring respiratory flow with differential pressure flowmeters under diverse inlet conditions through improved sensor configurations.
2. State of the Art
U.S. Pat. Nos. 4,859,858 ("the '858 patent") and 4,859,859 ("the '859 patent"), issued Aug., 22, 1989 to Knodle et al.; and U.S. Pat. No. 5,153,436 ("the '436 patent"), issued Oct. 6, 1992 to Apperson et al., each disclose apparatus including analyzers for outputting a signal indicative of the concentration of a designated gas in a sample being monitored by each apparatus.
The gas analyzers disclosed in the '858, '859, and '436 patents are of the non-dispersive type. They operate on the principle that the concentration of a designated gas can be measured by passing a beam of infrared radiation through the gas and ascertaining the attenuated level of the energy in a narrow band absorbable by the designated gas. This process is accomplished using a detector capable of generating a concentration proportional electrical output signal.
One important application of gas analyzers is monitoring the level of carbon dioxide in the breath of a medical patient. This is typically done during a surgical procedure as an indication to the anesthesiologist of the patient's condition. Of course, such gas analyzers can also be used by doctors in numerous medical procedures, such as heart stress tests with a patient on a treadmill, and the like.
In a typical instrument using non-dispersive infrared radiation to measure gas concentration, infrared radiation is emitted from a source and focused into a beam by a mirror. The beam is transmitted through a sample of the gases being analyzed. After passing through the gases, the infrared radiation beam passes through a filter. The filter reflects all of the radiation except for the radiation in a narrow band which corresponds to a frequency absorbed by the gas of interest. This narrow band of radiation is transmitted to a detector which produces an electrical output signal proportional in magnitude to the magnitude of the infrared radiation impinging upon it. Thus, the radiation in the band passed by the filter is attenuated to an extent which is proportional to the concentration of the gas of interest. The strength of the signal generated by the detector is consequently inversely proportional to the concentration of the gas of interest.
In typical medical gas analyzers, a cuvette is used to sample a patient's gas exchange via a nasal cannula or by being placed between an endotracheal tube and the mechanical ventilator. The cuvette channels respirated gases to a specific flow path and provides an optical path between an infrared radiation emitter and an infrared radiation detector, both of which can be detachably coupled to the cuvette.
A typical cuvette is molded from a polymer or other appropriate material and has a passage defining the flow path for the gases being monitored. The optical path crosses the flow path of the gases through windows in the sidewalls of the cuvette aligned along opposite sides of the flow passage allowing the beam of infrared radiation to pass through the cuvette.
The windows are generally formed from sapphire because of sapphire's favorable optical properties. However, sapphire is a relatively expensive material. Consequently, these cuvettes are almost invariably cleaned, sterilized, and reused. The cleaning and sterilization of a cuvette is time-consuming and inconvenient, and the reuse of a cuvette may pose a significant risk of contamination, especially if the cuvette was previously used in monitoring a patient suffering from a contagious and/or infectious disease.
Efforts have been made to reduce the cost of cuvettes by replacing the sapphire windows with windows fabricated from a variety of polymers. One of the major problems encountered in replacing sapphire cuvette windows with polymer windows is establishing and maintaining a precise optical path through the sample being analyzed. This is attributable to such factors as a lack of dimensional stability in the polymeric material, the inability to eliminate wrinkles in the windows, and the lack of a system for retaining the windows at precise locations along the optical path.
U.S. application Ser. No. 08/300,146, hereby incorporated herein by reference, discloses a cuvette and a method of manufacturing same which eliminates the problems encountered in previous attempts to use polymers in the place of sapphire windows. The application discloses fashioning a window from a malleable homopolymer such as biaxially oriented polypropylene in the thickness range of 0.001 to 0.005 inches. The use of this inexpensive polypropylene material allows for the fabrication of single use, disposable cuvettes.
Respiratory flow measurement during the administration of anesthesia in intensive care environments and in monitoring the physical condition of athletes and other individuals prior to and during the course of training programs and medical tests provides valuable information for assessment of pulmonary function and breathing circuit integrity. Many different technologies have been applied to create a flowmeter that meets the requirements of the critical care environment. Among the flow measurement approaches which have been used are:
1) Differential Pressure--measuring the pressure drop or differential across a resistance to flow. PA0 2) Spinning Vane--counting the revolutions of a vane placed in the flow path. PA0 3) Hot Wire Anemometer--measuring the cooling of a heated wire due to airflow passing around the wire. PA0 4) Ultrasonic Doppler--measuring the frequency shift of an ultrasonic beam as it passes through the flowing gas. PA0 5) Vortex Shedding--counting the number of vortices that are shed as the gas flows past a strut placed in the flow stream. PA0 6) Time of Flight--measuring the arrival time of an impulse of sound or heat created upstream to a sensor placed downstream.
Each of the foregoing approaches has various advantages and disadvantages, and an excellent discussion of most of these aforementioned devices may be found in W. J. Sullivan; G. M. Peters; P. L. Enright, M. D. "Pneumotachographs: Theory and Clinical Application," Respiratory Care, Jul. 1984, Vol. 29-7, pp. 736-49, and in C. Rader, Pneumotachography, a report for the Perkin-Elmer Corporation presented at the California Society of Cardiopulmonary Technologists Conference, October 1982.
At the present time, the most commonly used device for respiratory flow measurement is the differential pressure flowmeter. The relationship between flow and the pressure drop across a restriction or other resistance to flow is dependent upon the design of the resistance; thus many different resistance configurations have been proposed. The goal of many of these configurations is to achieve a linear relationship between flow and pressure differential.
In some prior art differential pressure flowmeters (commonly termed "pneumotachs"), the flow restriction has been designed to create a linear relationship between flow and differential pressure. Such designs include the Fleisch pneumotach in which the restriction is comprised of many small tubes or a fine screen, ensuring laminar flow and a linear response to flow. Another physical configuration is a flow restriction having an orifice variable in relation to the flow. This arrangement has the effect of creating a high resistance at low flows and a low resistance at high flows. Among other disadvantages, the Fleisch pneumotach is susceptible to performance impairment from moisture and mucous, and the variable orifice flowmeter is subject to material fatigue and manufacturing variabilities.
Most all known prior art differential pressure flow sensors suffer deficiencies when exposed to less than ideal gas flow inlet conditions, and further possess inherent design problems with respect to their ability to sense differential pressure in a meaningful, accurate, repeatable manner over a substantial dynamic flow range, particularly, when it is required for the flow sensor to reliably and accurately measure low flow rates, such as the respiratory flow rates of infants.
U.S. Pat. No. 5,379,650, issued Jan. 10, 1995 to Kofoed et al., hereby incorporated herein by reference, has overcome the vast majority of the problems with differential pressure flow sensors with a unique sensor including a tubular housing containing a diametrically-oriented, longitudinally extending strut containing first and second lumens having longitudinally-spaced pressure ports opening into respective axially-located notches at each end of the strut.
Developments in patient monitoring over the past several decades have shown that concurrent measurements of exhaled gas flow rate and CO.sub.2 concentration provides information that is useful in therapy decision making. By combining these two measurement, one can calculate CO.sub.2 production (V.sub.CO2) which is related to the patient's metabolic status. Also, these measurements can provide a graphical representation of the expired CO.sub.2 concentration versus expired volume which provides information about gas exchange in different compartments of the lungs.
Presently, the apparatus necessary to acquire the combination of these two signals requires two discrete components: a flow sensor (pneumotach) and a CO.sub.2 sensor. This configuration is cumbersome and adds undesirable volume (dead space) and resistance to the patient's breathing circuit.
It would be highly desirable to have an airway adapter which combines both a CO.sub.2 concentration monitoring sensor and a respiratory flow monitoring sensor in a configuration which is convenient to use and which minimnizes phase lag and internal dead space of the combination.